Forces loyal to Muammar Gaddafi pounded Misrata on Sunday, hours after the Libyan government claimed its troops had pulled back from the besieged city to let tribal leaders try to negotiate a political resolution, or unleash a "bloody" assault.

Using multibarrelled rocket launchers and tanks positioned at the edge of Misrata, loyalist troops fired hundreds of missiles at the city. At least six people, most of them civilians, were pronounced dead at the main hospital by midday. Dozens of injured were treated. "There has been a lot of shelling," said Anas Rajab, a doctor. "It looks like today will be another crisis day."

As the attacks continued, Mustafa Abdel Jalil, head of the Libyan rebel national council, said that Kuwait is to contribute 50m dinars (£110m) to the rebel council.

The renewed bombardment contradicted a claim by Libya's deputy foreign minister, Khaled Kaim, that "armed forces have ceased operations". The move, he said, was to give a 48-hour window for tribal leaders from the region south of Misrata to negotiate with the rebels over access to the port. If no deal could be reached by Monday night, the deadline, the tribes would launch an assault to "liberate" the city, Kaim said.

Tribal leaders have not confirmed any intervention, and rebel leaders in Misrata are sceptical about the government's statements. But Kaim said early on Sunday that the tribes were "trying to get in contact with the rebels".

The government's assertion that tribal leaders are ready to intervene politically and militarily may be an attempt to pressure rebels after the killing and capture of government troops, and to deflect attention from rebel gains.

Many rebels expect Gaddafi to make another strong attempt to take the city, but they dismiss claims that the tribes will be sent in instead of soldiers. Ahmed Mohamed Said, a computer engineer turned rebel, said it was a ruse to make the conflict look like civil war, rather than a government turning on its own people, and thus prevent Nato from assisting the rebels from the air. "Gaddafi wants it to look like brothers are fighting brothers," he said. "That will never happen."

Gaddafi's forces have faced setbacks in Misrata and the western mountain region, close to the Tunisian border. And Kaim acknowledged that loyalist troops had failed to take control of the port city after two months of siege. He said: "The tactic of the army was to have a surgical strike but, with the [Nato] air strikes, that doesn't work."

He added: "The leaders of the tribes decided to do something to bring normal life back. Their main demand is that foreign fighters leave the town or surrender themselves to the army."

The regime maintains that rebels fighting in Misrata and the east of the country are being driven by al-Qaida and Hezbollah militants – a claim rejected by the opposition.

Kaim said the tribal leaders want to reopen access to the port, which has been under the control of rebel forces since the siege began. The port served all Libyans, he said, but was of particular interest to tribes south of Misrata.

"The leaders of the tribes are determined to find a solution to this problem within 48 hours," he added. "The other option is military intervention."

He claimed the six tribes in the region could muster a force of 60,000 men to "liberate" the city. Any assault by the tribal forces would be ruthless, he claimed.

"The tribal leaders are pushing to intervene militarily," he said. "We have to do our utmost to stop this. If the tribes move into the city, it will be very bloody, and I hope to God we will avoid this."

On the ground, the rebels expanded on their gains of recent days. They said a large hospital under renovation in the city centre, which was used by Gaddafi's forces as a base, is now clear.

A small number of government troops appeared still to be hiding out in residential buildings and gun battles could be heard around the city. But most of the government forces and heavy weaponry remain concentrated beyond Misrata's southern highway.

Late last night three Libyan state television stations went briefly off air after three loud explosions were heard in central Tripoli soon after midnight. Libyan Television, Jamahiriya and Shababiya all stopped broadcasting but returned to air within half an hour.

In areas recaptured by rebels over the past two days, there is evidence of a crushing battlefield defeat of regime forces. In the city's main market, a large covered area open on the sides, there are six destroyed tanks and a burning petrol tanker. Thousands of machine-gun shells litter the ground, along with several cases used to carry mortar bombs. Most of the buildings nearby are badly damaged or destroyed by gunfire or shells.

Further south, near the technical college that was the scene of fierce fighting on Saturday, two large houses that had been occupied by Gaddafi's troops for a fortnight were black with smoke. At least 16 government soldiers were killed there, a rebel fighter said. Two charred bodies lay in the living room.

On the southern highway, controlled by the government until Sunday, a tank had nosedived into a trench dug across the road by the rebels. A burnt pickup truck was nearby, its cargo of rocket-propelled grenade launchers smouldering next to it. Green uniforms discarded by fleeing soldiers had been flung nearby.

"There was no retreat, just fighting, and we forced them back," said Hassan Mohamed, a rebel fighter.